The hydraulically actuated valve uses the oil pressure created by the actuator pump to open the valve against the air pressure used for quickly closing the valve. In the expanded condition the spindle elongates slightly causing the oil in the actuator cylinder to push the roller down closer to the cam base circle

In essence the effect is the same as the condition of the mechanical push rod where the roller clearance reduces in the hot running condition as compared to the cold stationary condition. Hence the roller clearance has to be checked in the hot running condition and adjusted to the makers recommended value for hot condition. This has to be done after every valve replacement or overhaul.